This competing renewal application proposes to study the relationship between antibody structure, antigen specificity and protective efficacy for monoclonal antibodies (mAbs) to the fungus Cryptococcus neoformans. The past funding period has been an exciting time of discovery marked by the findings that specificity and isotype are crucial determinants of antibody function against C. neoformans. The focus of this proposal is the question of how antigen specificity contributes to antibody protective efficacy against a fungal pathogen. In the past funding period, Dr. Casadevall s laboratory has discovered that two IgMs derived from one B cell differ in fine specificity, binding to the capsule and protective efficacy. The experimental approach is to use site directed mutagenesis coupled with functional assays to dissect the protein structural features that are responsible for specificity and function. Three specific aims are proposed: 1. To identify the amino acid residues responsible for epitope specificity in the closely related mAbs 12A1 (protective) and 13F1 (non-protective). 2. To establish their relationship between structure, specificity and in vitro and in vivo for mAbs 12A1 and 13F1 and their site-directed mutagenized derivatives. 3. To determine the mechanism of IgM function against C. neoformans with emphasis in understating how specificity contributes to protective efficacy. On a practical level, Dr. Casadevall s laboratory hopes to use this information in design of therapeutic antibodies and vaccines against C. neoformans.